The technological background is illustrated by U.S. Pat. No. 3,268,605, which describes a process for controlling the composition of the fluids in a simulated moving bed.
The reactors or adsorbers that are used now are increasingly large in order to meet a rising demand for the product in question.
Furthermore, the desired product should reach a purity that most often exceeds 99.5%, which is not a priori compatible with the volume of the feedstock that is to be treated and therefore with very large reactor capacities.
The technological background that illustrates the implementation of an adsorption device with simulated countercurrent is described in, for example, U.S. Pat. No. 2,985,589. This device comprises at least one cylindrical column that contains a solid mass that is cylindrical overall and is essentially annular in cross-section. A main fluid that is introduced by a pump flows through the solid bed along the central axis of the column according to a flow that we wish to describe as a piston-type flow (plug flow). In other words, the fluid should have a composition and flow front that are uniform at all points of the section of the column.
A device such as the one that is described in U.S. Pat. Nos. 3,214,247 and 4,387,292, which are incorporated as references, makes it possible to achieve this goal. This device generally comprises a number of beds of an adsorbent, fed by a number of distributor plates, whereby each bed is supported by an upper grid that is approximately perpendicular to the axis of the reactor and makes it possible for the fluid to flow. Each plate is divided into sectors, and each distributor plate segment comprises two deflectors that are non-perforated, flat, or tapered overall (of variable thickness) arranged on the same horizontal plane, between which a circulation space for the fluid is placed. A lower grid under the deflectors makes it possible to distribute the fluid uniformly in the lower adsorbent bed.
At each distribution plate, at least four transfer lines for secondary fluids (feedstock injection line, desorbent injection line, draw-off line of an extract, and draw-off line of a raffinate) that contain a set of valves are connected to means of switching this set of valves.
The injections and draw-offs of these fluids are carried out between some beds that define zones, and over the length of time that is called period T the introduction and draw-off points that delimit the zones of between-bed interval (c.sub.k) and (c.sub.k+1) are shifted to between-bed interval (c.sub.K+1) and (c.sub.k+2). Cycle time Tc is defined as being the length of time that separates two consecutive injections of the same fluid onto same plate P.sub.i. Length of time T can vary from one bed to the next.
A recycling pump recycles the fluid from the low end of the column to the top end of the column.
The secondary fluids (feedstock or desorbent) are introduced or drawn off (extract, raffinate) into or from the circulation space via an introduction or draw-off chamber that is pierced with holes.
Each distributor plate can be divided into sectors. According to U.S. Pat. No. 3,789,989, each plate sector, delimited by radial walls, comprises a chamber for the introduction or draw-off of the secondary fluid.
In the case where the distributor plate of each sector comprises only a single chamber, each chamber of a given sector is connected via a tube to a single feed line or draw-off line that is connected to the outside of the column.
According to Patent Application EP-A-769316, each secondary fluid is introduced or drawn off via its own introduction or draw-off chamber that has a number of orifices opposite the circulation space. The upper and lower walls of these chambers constitute the deflectors that are mentioned above. Therefore, when the distributor plate of each sector comprises several chambers, each chamber of a bed sector is connected via a tube to a line that is intended to receive only a single fluid or to feed the corresponding chamber with desorbent or with feedstock or to draw off the raffinate or the extract from the appropriate chamber. Thus, for example, if each sector comprises four chambers, one intended for feedstock, the second for desorbent, the third for raffinate, and the fourth for extract, the CF chamber of the given sector that receives feedstock F will be connected to a line that receives all of the pipes of different chambers CF associated with the same adsorbent bed.
In a paraxylene separation unit that operates in a simulated moving bed and that comprises two adsorbers that are arranged in a series of a dozen sieve beds each, deformation (or drag) of the longitudinal concentration profiles, which is reflected by deficient performance relative to the desired ideal performance, has been noted.
In particular, the drag of the impurity concentration at the extract draw-off level is reflected by a significant reduction in the purity of the extract (less than 99%) compared to the desired purity (greater than 99.5%). An analysis of the problem, carried out on the separation unit, showed that these deformations (or drags) of the longitudinal concentration profiles were due to parasitic circulations through each of the distribution chambers that are arranged on the sectors of each plate during periods when there is neither introduction nor draw-off of fluid through the chamber in question. This is in particular a wow, i.e., a material exchange caused by turbulence at the orifices of the distribution chambers between the main fluid that circulates in the circulation space and the fluid that is contained in the chambers. This phenomenon is known for producing small drags. It is also especially recirculation of a distribution chamber of a plate sector toward the similar chamber of another sector of the same plate, via the coupling pipe that connects these chambers to one another and to the transfer line to the outside of the adsorber.
This recirculation is caused by small pressure differences that exist between the sectors of the same plate. In theory, this pressure should be the same everywhere on the same plate. In practice, small differences exist because of various imperfections such as the imperfections of flow of the main fluid through the adsorbent beds, and this induces, during the periods where there is neither introduction nor draw-off of secondary fluid in a chamber, the recirculation of a portion of the main fluid that is withdrawn at the circulation space of a sector where the pressure is higher to the circulation space of a sector where the pressure is lower, whereby this occurs via the orifices of the chambers in question. The portion of the main fluid that is recirculated enters one of the chambers by passing through the orifices of the chamber that is affected and that belongs to the higher-pressure sector. This portion of fluid then advances toward the similar chamber that belongs to the lower-pressure sector via the coupling pipe that connects these chambers to one another. Finally, this fluid portion rejoins the main fluid in the circulation space of the lower-pressure sector by passing through the orifices of the chamber of this sector.
The recirculation flow rate between two sectors of the same plate is a function of the pressure differences that exist between these two sectors, as well as the sizes of the orifices of the chambers of the sectors that are affected. The dwell time of the fluid that thus recirculates from one chamber of a sector to the corresponding chamber of another sector is itself a function of the volume of the exit and arrival chambers, the pipe volume that connects them, and the recirculation flow rate between these chambers. If the plate comprises multiple sectors, there will be a combined general recirculation from the sectors where the pressure is the highest toward the sectors where the pressure is the least high, whereby this recirculation takes place with a mean overall dwell time TR.
In this unit that operates on a simulated moving bed, the composition of the main fluid at the level of a plate changes constantly as a function of time. This is because of the advance of the longitudinal concentration profile, which moves under the action of the circulation of the main fluid. Taking into account the parasitic recirculation that is observed, it follows that on a plate taken at a given instant there arrive, on the one hand, the main fluid that has a given composition and, on the other hand, the portion of the recirculated main fluid that is recirculated from a portion of the sectors toward the other sectors of this same plate that has a composition that corresponds to that which the main fluid had a moment before, whereby the time shift is equal to dwell time TR of the portion of recirculated fluid. Everything therefore happens as if a portion of the main fluid came to each plate with a certain delay, which is equal to dwell time TR. The mixing of this portion of fluid that is recirculated with a delay with the main fluid modifies the overall composition of the combined fluid and therefore causes a systematic retromixing at each plate. This induces a deformation, or drag, of the longitudinal concentration profiles and is reflected by a loss in performance as the reduction in purity of the extract that can reach, for example, up to a point.